No keyboard backlight. F5/F6 keys work, dimmer status shows on screen

I just picked up a 2009 macbook pro 17. every time i purchase a macbook, i thoroughly check several functions, including the backlit keyboard. considering how often i purchase MacBooks, i don't have a vivid recollection of testing this MacBooks keyboard backlight. i'm pretty sure it checked out, because i usually pick up around 3/week and i know what does and doesn't work.

i slapped an SSD in this book, and installed el capitan (it was running yosemite), and noticed they keyboard backlight wasn't working. i deselected ambient light sensor in keyboard properties. when i press F5/F6, the brightness indicator changes, but the backlight never turns on. i popped off the bottom cover and connected (temporarily) another backlight, and it didn't turn on either.

i've tried reseating the connection multiple times, and resetting the SMC and PRAM after every adjustment.

i had another 15" 2009 where it was solved simply by fidgeting with the connector and resetting the SMC, but this one just won't go.

ok edit: i tried fussing with the ribbon cable an infinite amount of times. i put the original hard disk with yosemite back in, to no avail. i swapped out the ram, to no avail.

i went to put my voltmeter on the backlight ribbon, but i got no voltage across the terminals.

does anybody know what the voltage should read, and if left or right is positive? my fluke is auto sensing, and i tried reversing poles, but never got any voltage. i'm unsure if there's a bad connection at the socket, or if the logic board is bad. i don't have any leads/probes/clamps which are small enough to safely test the socket.

i know that apple techs don't even repair these parts, they just replace them, but still i'll ask...

does anybody have (or know how to find) any schematic info for these things?

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